Antioxidants ("AO"s) are used to protect hydrocarbon polymers against thermal oxidative ("thermooxidative") degradation caused by reaction with atmospheric oxygen, usually at, or above ambient temperature. Oxidation causes undesirable changes in mechanical, aesthetic, and electrical properties of the polymer. Some polymers degrade more readily than others, depending upon their structure. The high order of effectiveness of several tetramethylpiperidinyl benzoate derivatives in inhibiting the thermal oxidation of lauryl aldehyde was measured and the data reported in an article titled "Stabilization Mechanisms of Hindered Amines" by Peter M. Klemchuk and Matthew E. Gande in Makromol. Chem., Makromol. Symp. 28, 117-144 (May 1989). The rate of oxygen uptake with no stabilizer was 3.6 mmol/hr; with BHT was 1.9 mmol/hr; and with tetramethylpiperidin-1-oxyl was 0.08 mmol/hr. The rate of oxygen uptake was comparably low (that is &lt;0.1 mmol/hr) for tetramethylpiperidinyl compounds with &gt;N-H; &gt;N-OH; and &gt;N--O.degree. groups in the tetramethylpiperidinyl moiety, indicating that stabilizers containing a tetramethylpiperidinyl moiety and known to be good light stabilizers, have good thermooxidative properties.
Despite the piperidinyl derivatives being generally regarded as ultraviolet light stabilizers, such known, good thermooxidative stabilization obtained with the piperidinyl derivatives, led to the statement: "In recent years, interest in the use of antioxidants to give protection against the effects of light has considerably increased. There are a number of reasons for this, but the main ones appear to be advances in the understanding of mechanisms of photo transformations of polymers of different kinds, which demonstrate the key role of free radicals and photochemically active molecular intermediate products with which antioxidants are likely to interact. From the practical viewpoint a significant incentive proved to be the development of new high-efficiency light stabilizers, especially the sterically hindered amines, which act predominantly as antioxidants. Interest in the use of antioxidants for light stabilization is also associated with the fact that they happen, at the present time, to be the principal class of thermostabilizers of polymers. Consequently, their choice makes it possible to achieve complex protection of the polymeric material. See "Stabilization of Polymers Against the Effect of Light by Means of Antioxidants" by V.B. Ivanov and V. Ya. Shlyapintokh Polymer Deoradation and Stability 28 (1990) 249-273.
The foregoing statement, despite the fact that hindered amines are usually classified as light stabilizers rather than AOs (since they are extremely effective in protecting polyolefins and other polymeric materials against photodegradation) underlines the intense search in the marketplace for a hindered amine which is both an AO and a UV stabilizer.
Though the piperidyl compounds are stated to be effective antioxidants, they are not effective enough (as an antioxidant) in polyolefins, and in particular, homopolymers of propylene, or copolymers of propylene and ethylene in which the latter is present in a minor amount by weight (such homopolymers and copolymers are collectively referred to herein as "PP" for brevity), to warrant their use, without the use of a secondary stabilizer, for such duty. When piperidinyls are used as stabilizers, they are used as light stabilizers to protect against ultraviolet radiation, and, in practical applications, only in combination with a phosphorous acid ester, hindered phenol or other antioxidant.
The foregoing is equally true for piperazin-2-one-containing compounds used as hindered amine light stabilizers ("HALS" for brevity). Evidence of the foregoing is provided in an article titled "Hindered Diazacycloalkanones as Ultraviolet Stabilizers and Antioxidants" by J.T. Lai, P.N. Son and E. Jennings, Polymer Stabilization and Degradation 91-99, ACS Symposium Series, American Chemical Society, Washington, D.C. (1985). Slit tapes of PP (2 .times. 100 mil) containing a combination of 0.1 phr (parts per 100 parts of substrate) of HALS, aged in the Arizona sun, even the polymeric piperidine did not fare very well (pg 92).
Unique among the HALS disclosed in the aforesaid article was a decahydroquinoxaline which provided both AO and light stabilization. It was determined that the most important features for its AO activity were (i) the fused bicyclodiazacycloalkane ring, (ii) dialkyl substituents at the C.sup.3 position, and (iii) alkyl, instead of acyl substituents at the N.sup.1 position.
We have now provided a monomeric N.sup.4 -methylated oxopiperazinyl triazine (MPIP-T) which, by itself, gives protection against thermooxidative degradation when essentially homogeneously distributed in a wide array of synthetic resinous materials. In addition, it is an excellent stabilizer against degradation by UV light, and provides adequate stability during melt processing. No known HALS stabilizer provides this combination of properties.
The conventional view of the HALS in general, and the polysubstituted piperazinone-containing ("PSP") HALS in particular, was that such contribution to thermooxidative stabilization as they made in combination with a hindered phenol AO or other secondary stabilizer, was eschewed in favor of leaving out the AO for the specific purpose of obtaining better gas fade stabilization and stabilization against gamma-radiation. U.S. Pat. No. 4,797,438 patent teaches the gamma-stabilizing activity of PSP-containing HALS in the absence of an antioxidant; and U.S. Pat. No. 4,929,653 teaches the ability of PSP-containing HALS to improve gas fade resistance in the absence of an antioxidant.
A recent article titled "New Developments in Polymer Stabilization" by Motonobu Minagawa Polymer Degradation and Stability 25 (1989) 121-141 provides an up-to-date appraisal of the field as viewed by one skilled in the art. At the outset it points out that, since chemical reactions of stabilizers contribute to stabilization, these reactions proceed in the molten highly viscous state or in the amorphous solid phase. Additives should, therefore, possess adequate mobility to react in plastics. This is probably the reason why the molecular weights (MW) of commercial stabilizers are almost always in the range of 200-2000, although there are some exceptions. But to prevent loss of stabilizer from the polymer in increasingly severe conditions of application, the tendency is to increase MW.
Among the low MW HALS in the range from about 400-1000 are Tinuvin 144 (Ciba-Geigy), Sanol LS 2626 (Sankyo), Mark LA-57, Mark LA-52 and Mark LA-62 (Adeka Argus), and Sumisorb TM-061 (Sumitomo Chem). But such low MW HALS result in loss or decomposition during the period from processing to end use. For the high retention which is necessary, high MW HALS are used.
Among the high MW HALS in the range above about 2000 are Tinuvin 622 LD, Chimassorb 119 and Chimassorb 944 (Ciba-Geigy); Mark LA-63 and Mark LA-68 (Adeka Argus); Spinuvex A-36 (Montedison) and Cyasorb UV-3346 (American Cyanamid).
But all HALS tend to form salts with acids and the salts adversely affect weather resistance, hence greatly reduce the performance of the stabilizer. Moreover, the salts contribute to contamination of the surfaces of molds or dies used to mold or extrude, or otherwise thermoform, the articles to be stabilized. Such problems associated with the alkalinity of the piperidinyl-containing HALS have been substantially negated by the use of a HALS with one or more piperazin-2-one moieties. Especially desirable stability is obtained with piperidyl moieties connected to a triazine ring, such as Chimassorb 944 and Chimassorb 119 commercially available from Ciba-Geigy. Comparable or better stability is obtained with piperazin-2-one moieties distally connected to a triazine ring in the stabilizers disclosed in U.S. Pat. Nos. 4,480,092 to Lai et al., and 4,629,752 to Layer et al, both in class 544/subclass 113.
We have now provided a method for molding or extruding articles melt, and thermooxidatively stabilized with only a low MW HALS, specifically a MPIP-T, while maintaining the surfaces of the mold or die essentially free of contaminants during operation.
As stated hereinabove, HALS protect polymers by functioning as light-stable antioxidants. Their antioxidant activity is explained by a reaction sequence in which hindered amines terminate propagating reactions by trapping both alkyl and peroxy radicals. But oligomeric HALS are highly effective against oxidation when exposed at 120.degree. C. in a forced-air oven. (see "Antioxidants" Vol 2, Encyclopedia of Polymer Science and Engineering, 2d edition, John Wiley & Sons). A piperazinone-containing HALS which we use, by itself without a secondary stabilizer (hence referred to as a "virgin MPIP-T"), has unsubstituted N.sup.4 atoms, is not oligomeric and has a MW &lt; 1000. Yet the MPIP-T has a very low rate of migration. The rate of migration is so slow that the stabilizer does not cause significant contamination of a mold--a problem which must be addressed when the stabilizer is combined with a hindered phenol. Neither is the stabilizer readily extractable with ethanol in which the stabilizer is typically soluble. Such low extractability makes the MPIP-T stabilizer acceptable for use in containers for comestible foods. This MPIP-T is also relatively non-toxic having a toxicity far less than that of the commercially used hindered phenols, and comparable to that of a piperidnyl-triazine containing HALS.
Compounds referred to as oxo-piperazinyl triazines in this specification are those which contain a triazine ring with three piperazinone substituents, each of which has a alkyl-substituted N.sup.4 atom, and each of which is substituted with two substitutents, or, substituted with a spiro substituent, (referred to as being "polysubstituted"), at both the 3 and 5 positions of the piperazinone ring. Compounds referred to as piperidinyls are those which contain at least one piperidinyl ring which is either disubstituted, or substituted with a spiro substituent at the 2 and/or 4 positions. When substituted as described, such compounds, whether piperazinones or piperidines, are referred to as being polysubstituted. Compounds referred to as MPIP-Ts are those which contain only a single triazine ring having a substituent at each C atom of the ring, each substituent containing a polysubstituted piperazinone ring.
A monomolecular virgin MPIP-T having a single triazine ring, has been found to be a better AO in polyolefins, particularly in PP, than an oligomeric stabilizer having a triazine ring connected to plural polysubstituted piperidyl moieties. The emphatic interest in using a virgin PIP-T is to avoid the complicating side-effects which attend the use of secondary stabilizers, and to lower cost.
The PSP moieties of the MPIP-T are connected to the triazine ring through an alkyleneamino chain. In prior art oxo-piperazinly triazines (PIP-Ts) disclosed in the '092 patent, the chain is unbranched; and in the '752 patent, the chain is branched. Details of the structures of the MPIP-T are set forth hereafter.
We have now found (as will be evident from the data set forth hereunder) that the virgin MPIP-T which we knew provided excellent stabilization against degradation by light (that is, had excellent u-v light stabilization properties), also functions as an excellent AO when used in a concentration no more than 0.2 phr, without any secondary stabilizer. As an AO, the virgin MPIP-T is a more effective AO than not only the conventional hindered phenols which dominate the market-place, but also more effective than the dominant HALS oligomeric stabilizer Chimassorb 944 which is a compound having a repeating unit consisting essentially of a triazine ring and two polysubstituted piperidine rings.
Because there is so small a concentration of virgin MPIP-T in a stabilized polyolefin the MPIP-T is far less prone to migration to the surface of the article, particularly during molding or extruding the polyolefin, thus minimizes the contaminant effect of the MPIP-T on the surfaces of the mold or die. Such a low concentration also makes it difficult to extract the MPIP-T with a solvent, thus permits using the MPIP-T in polyolefins for making containers for comestible foods, because such very small amount of MPIP-T as may be extracted into the food, renders de minimis the slight toxicity of the MPIP-T.
The unexpected properties of the virgin MPIP-T deriving from its use as an AO in synthetic resinous materials are attributable to the presence of the PSP moiety, and the fact it is distally connected to the triazine ring by an alkyleneamino chain. We have now found that this structure accounts for the easily distinguishable AO activity of a monomeric virgin MPIP-T compared with that of a piperidyl-triazine oligomer.
In addition to the surprising effectiveness of a MPIP-T when it is used as the only stabilizer in any synthetic resinous material, whether in fiber or any other shaped article, the MPIP-T is extremely persistent--that is, it does not degrade thermally. Nor is a MPIP-T volatilized during the manufacture of an article despite its essentially similar volatilization temperature compared with that of Chimassorb 944. Nor is the MPIP-T volatilized during the useful life of the article at relatively high temperature, which, if the article is made from PP, is a temperature in the range from about 120.degree. C. to about 145.degree. C., or below the heat distortion temperature (HDT) of the PP.
This resistance to migration at elevated temperature makes a MPIP-T the stabilizer of choice for impellers for washing machines for clothes, and impellers for dish-washing machines; as an inner liner for automobile fenders; for under the hood applications in automobiles; etc.
Further, a MPIP-T is exceptionally compatible with polymers, and in the low concentration used in this invention, behaves as a single phase, for example by being substantially soluble in many polymers, particularly PP, polyethylene (PE), and copolymers thereof, the solubility being affected by the molecular weight of the polymer. This is particularly note-worthy because polar UV stabilizers are known to be excluded from the crystalline phase of a polyolefin such as PP, preferring to be concentrated in the amorphous phase from which the stabilizer can more readily migrate to the surface of a mold or die in which an article is being formed.
Equally noteworthy is that most HALS with a substituent on the N atom, result in retarded nitroxide formation. This was studied in piperidyl amines and reported in an article titled "Photo-degradation and Photo-stabilization in Organic Coatings Containing a Hindered Amine Light Stabilizer: Part VI--ESR Measurements of Nitroxide Kinetics and Mechanism of Stabilization" by D.R. Bauer et al Polymer Degradation and Stability Vol 28, 115-129 (1990. Conversion was fastest for the &gt;N--H group. Also fast is that for &gt;N--CH.sub.3, each of which is about an order of magnitude faster than the conversion for other substituents. This provides an insight into the AO activity of the virgin MPIP-T.
Moreover, preferred MPIP-Ts are essentially water-white, or only slightly colored, crystalline or amorphous solids, or non-viscous liquids, so that they are easily homogeneously dispersed in a polymeric substrate and impart no color, or only a negligible amount of color to the substrate in which they are dispersed or dissolved. Because of its bulky structure which is remarkably soluble as long as the concentration is no more than 0.2 phr, particularly in PP, once compatibilized in the substrate, a MPIP-T diffuses ever so slowly. In addition, its relatively lower pH compared with piperidinyltriazines, appears to negate the tendency of the MPIP-T to contaminate the surface of molds or dies under injection molding, blow molding, or other high temperature forming operations. This not only avoids marring the surface of the article formed, but avoids having to interrupt operation of the molding machine to clean the surfaces of the cavities in the mold frequently.
Perhaps, most important, the properties of a MPIP-T allows it to be used to stabilize housings for equipment,; fibers for fabrics for clothing, even articles worn next to the skin; receptacles such as garbage cans, and containers for foods and beverages for human consumption, because when used in the amount of no more than 0.2 phr in polyolefins, this MPIP-T is typically not objectionably toxic.